The proposed experiments are designed to yield new information on the in vivo development and function of neoplastic pituitary tumor cells of MtTW15 mammosomatotropic tumors which secrete prolactin, growth hormone, and ACTH. Identification and characterization of cells which produce these hormones will be accomplished through the use of ultrastructural immunocytochemistry, and evidence on the single or multiple hormone-secreting capabilities of individual tumor cells will be obtained. The development of tumor cell populations secreting prolactin, growth hormone, and ACTH will be characterized by use of quantitative immunocytochemistry using linear scanning techniques at the light microscopic level. The percentage of the tumor cell population producing hormones during development will be correlated with hormone levels of tumors and plasma as evaluated by radioimmunoassays. Quantitative immunocytochemistry will be used to evaluate changes in tumor cells after treatment with ergocornine and other drugs which are known to markedly alter hormone production by MtTW15 tumors to determine if these agents can be used to either initiate or suppress the in vivo differentiation of specific hormone-producing cell types. These studies will provide a model for accurately evaluating the effects of drugs of potential therapeutic value on experimental pituitary tumors. They will also provide insight into the mechanisms of alteration of neoplastic cell function and contribute to the development of monohormonal experimental tumors.